W. Sadowski

Raman spectra of single crystal CuO

Polarized Raman measurements on single crystals of CuO yield the symmetry assignments of the three predicted Raman active lattice modes : 297cm−1 (Ag, 344cm−1 (Bg) and 629cm−1 (Bg). These results are compared to literature data, including IR spectra. Our measurements confirm at low temperature the appearance of an additional Raman band around 240 cm−1. The temperature dependence of the linewidth of the Ag mode presents an anomalous behavior near the magnetic phase transition, suggesting the possible presence of magnon-phonon couplings in the antiferromagnetic phase.

Reproducible growth of large free crystals of YBa2Cu3O7 − x

Abstract By optimizing the experimental conditions for growth of yttrium barium cuprate (YBCO) from BaO-CuO solutions, free platelet crystals larger than 3 × 3 × 0.1 mm 3 and bulk separated crystals larger than 3 × 3 × 1 mm 3 could be achieved reproducibly. The thickness of the crystals depends on cooling rate, concentration of solution, impurities and (dissolved) crucible material.

Polaron imprints in the infrared spectra of Nd2CuO4−y

Abstract In addition to four Eu phonons, Nd2CuO4-y single crystals show far-infrared reflectivity peaks which depend on temperature and y. They provide evidence for the insurgence of local modes induced by doping. The so called d- or J-band at ∼ 1000 cm-1 can be partially resolved, and shown to be produced by combination bands of those local modes. These results provide direct evidence for polaron formation in this compound.

Growth of single crystals, thermal dependency of lattice parameters and Raman scattering in the Nd2-xCexCuO4-δ system

Abstract We report on the growth of Nd 2- x Ce x CuO 4- δ single crystals (0 x 2 O flux. Free separated crystals with maximum size of 5x8x0.15 nm 3 have been obtained. Magnetic AC susceptibility measurements show a sharp superconducting transition at temperatures up to 23 K. The temperature dependence of the lattice parameters has been measured by means of X-ray powder diffraction between 10 K ( a=3.9413(3) A , c=12.0290(18) A ) and 290 K ( a=3.9482(3) A , c=12.0590(18) A ). Room temperature Raman spectra reveal a new band at 320 cm -1 which is not observed in Nd 2 CuO 4 . Raman spectra of crystals with T c ranging from 7 to 22 K show a systematic intensity change of the broad band at 590 cm -1 .

Unusual behavior of the Gd ESR in single crystals of GdyY1−yBa2Cu3O6+x with x=0.1−0.8 and y=0.03−0.06: Evidence for magnetic interaction in the superconductors

The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of GdyY1−yBa2Cu3O6−x has been measured. In the insulating compound, with x∼0.1, and the superdconducting materials with 30 K < Tc < 80 K, the measurements were performed at X-band, 9.3 GHz, and Kα-band, 36 GHz, over a large temperature range above Tc. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actual Tc of 70 K. The crystal field parameters are D = 3B02 = 1307 MHz, B04 = 3.014 MHz and B44 = -11.43 MHz, for the semiconducting sample. The g-value is 1.989 0.005. These values change only slightly in the superconducting crystals.

Superconductivity in PrBa2Cu3O7-δ single crystals after high-temperature thermal treatment

Abstract The influence of post-growth treatment, consisting of high-temperature reduction, quench and oxidation, on the structural, electrical and magnetic properties of PrBa 2 Cu 3 O 7− δ single crystals, obtained in Al 2 O 3 and ZrO 2 crucibles by the self-flux method, was examined. We report on the observation of inhomogeneous superconductivity in several Al-doped crystals from the ac magnetic susceptibility and resistivity measurements. Superconducting crystals were characterised by X-ray diffraction technique (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Room-temperature scanning tunnelling microscopy (STM) was used to study the topography and local electric conductivity of the crystal surface after thermal treatment. We attribute the onset of the superconductivity in our samples to the structural changes after high-temperature reduction in argon atmosphere providing the decrease in number of Pr on Ba site defects and enhanced by the existence of Al-rich clusters in the material.

Single crystal ESR studies on tetragonal YBa2Cu3O6+x

We have characterized as-grown and thermally treated YBaCuO single crystals by ESR, Raman spectroscopy, magnetic susceptibility, X-ray and neutron diffraction measurements. The as-grown crystals are tetragonal and are superconducting with an onset temperature of 30 K. They show an ESR signal which behaves as a localized Cu2+ ion with tetragonal symmetry and presumably originates from copper chain atoms that are octahedrally coordinated by six oxygen neighbors. The temperature dependence of the ESR between 150 and 270 K shows paramagnetic behavior and also dynamical features. Below 90 K, the ESR signal disappears reversibly. A likely explanation is that the onset of local superconductivity frustrates the spins responsible for the ESR signal.

Crystal growth and physical properties of Nd2−xCexCuO4−y single crystals

Abstract Single crystals of Nd2−xCexCuO4−y with dimensions up to 5 × 8 × 0.15 mm3 were grown from Cu2O flux at 1250 °C. The effects of the reduction time at 900 °C in N2 atmosphere have been investigated. For more than 20 hours we found by EDAX analysis an increase of Ce and Cu concentration at the crystal surface and we simultaneously observe, by means of X-ray diffraction, the appearance of a cubic phase (Nd0.5Ce0.5O1.75) within the crystal. The best superconducting properties are obtained for a reduction at 900 °C for 10–15 hours with Tc = 22.9 K and ΔTc = 0.4 K. The resistivity shows a metallic temperature dependence, ρ(300K) = 550μΩcm and the Hall coefficient (H//c-axis) is negative between Tc and room temperature.

Comparison of Hall effect near Tc in YBCO 123 single crystal and 124 ceramics

Abstract We have measured the Hall voltage V H as a function of temperature and magnetic field B (up to 6 T) near T c in Y 1 Ba 2 Cu 3 O 7−δ (“123”) single crystal and in Y 1 Ba 2 Cu 4 O 8 (“124”) ceramics. Near T c , V H shows a sign reversal in the 123 crysta l ( B parallel to the c -axis) and the tangents to the V H versus B curves at 6 T do not cross the origin. These features are not observed in the 124 phase. The fact that a negative V H appears in the 123 phase and not in 124 seems to reflect different conditions for the flux flow dynamics in the two compounds. We also report measurements of the normal state Hall coefficient R H obtained in single phase 124 ceramics. The very small value of R H (1.5×10 -10m 3 /C for T > 140 K) is rather unusual in the superconducting oxide family.

Study of superconductivity in a wide range of Ce doping in Nd-214 system

We report properties of Nd 2-x Ce x CuO 4-y (NCCO) single crystals which were reduced using special reduction technique, described previously by Brinkmann et al. The superconducting phase diagram obtained for our Nd 2-x Ce x CuO 4-y single crystals is the first sign showing the existence of superconductivity beyond the region 0.13<x< 0.17 for Nd 2-x Ce x CuO 4-y . Pure and weakly cerium doped (less than 0.03) Nd 2 CuO 4 crystals annealed in the same conditions, completely decomposed. It is probably caused by more effective reduction process for small x than for the bigger. In this work we discuss the correlation between structure and superconducting properties in NCCO system after high temperature reduction with surface protection.